1. Field of the Invention
The present invention generally relates to a wavelength converter; and more particularly, to an optical wavelength converter for phase-modulated light signals for use, for example, in coherent (interferability) optical communication or with an all-light cross connect.
2. Description of the Related Art
In recent years, developments are in progress for wavelength division multiplex (WDM) optical communication schemes in large-scale optical communication systems. In the optical communication systems, the network performance can be significantly improved by converting the wavelengths of signals at nodes of the network. Conventionally, as a wavelength conversion device of a practical phase modulation scheme, there is generally known a device using four-wave mixing. Specifically, various wavelength converters have been proposed, utilizing, for example, cross-gain modulation or cross-phase modulation of semiconductor laser amplifiers or utilizing four-wave mixing of lights (see, for example, Patent Reference Document 1).
FIG. 5 shows an overall constitution of a conventional wavelength converter. In the wavelength converter for an optical phase modulation communication using a four-wave mixing, input phase-modulated light having a wavelength λ1 is mixed by an optical coupler with continuous wave light (referred to as “CW light” or “pump light”, hereinafter) having a wavelength λ3 generated by a laser diode (LD). Then, the mixed light is inputted to an optical medium such as a semiconductor optical amplifier (referred to as “SOA”, hereinafter) having nonlinear optical effects. In the SOA, a phase-conjugate wave having a wavelength λ2 (=2λ3−λ1) is generated by the four-wave mixing. The phase-conjugate wave having a wavelength λ2 is symmetric to the wave of the wavelength λ1 with respect to the wavelength λ3 on a wavelength axis. The phase-conjugate wave is outputted through an optical filter to thereby obtain wavelength-converted light having a wavelength λ2 that has undergone the same phase modulation as that for the wavelength λ1.
In another conventional art of, for example, Patent Reference Document 2, there is disclosed a wavelength converter of an electro-absorption (EA) type. In this art, incident signal light and continuous wave (CW) probe light from a probe light source are inputted to an electro-absorption (EA) type modulator. The probe light outputted from the electro-absorption (EA) type modulator has a waveform incorporating the waveform of the incident signal light, and is fed to a subsequent stage by a circulator.
In still another conventional art of, for example, Patent Reference Document 3, there is disclosed a wavelength converter for intensity-modulated light signals. This wavelength converter has, for example, a waveguide for generating nonlinear refractive index variations, a delay interference circuit with two optical paths having different optical path lengths, and a CW light source, thereby achieving an simplified constitution and control with a stable operation.
In yet another conventional art of, for example, Patent Reference Document 4, there is disclosed a wavelength converter for intensity-modulation light signals. This wavelength converter includes a monolithically integrated delay loop located in a delay interference construction having only a SOA, using an asymmetric isolation coupler.
Patent Reference Documents referred to in the above are as follows:
Patent Reference Document 1: Japanese Patent Unexamined Laid-open Publication No. 10-78595 (Paragraphs 0017 to 0010; FIGS. 11 to 14)
Patent Reference Document 2: Japanese Patent Unexamined Laid-open Publication No. 2000-236302 (Paragraph 0030; FIG. 1)
Patent Reference Document 3: Japanese Patent Unexamined Laid-open Publication No. 10-301151 (Paragraphs 0015 to 0020; FIG. 1)
Patent Reference Document 4: Japanese Patent Unexamined Laid-open Publication No. 2002-40504 (Paragraphs 0008 to 0010; FIG. 1)
Nevertheless, however, the conventional wavelength converters use four-wave mixing, which is one type of nonlinear optical effects obtained by such as a SOA. Therefore, there arises a problem that the intensity of the obtained wavelength-converted light is low in comparison to that of the CW light of the wavelength λ3, and the conversion efficiency is low accordingly.
A relation as expressed by the equation given below exists between the individual wavelengths λ1, λ2, and λ3:λ2=2×λ3−λ1
Therefore, it is impossible to convert the input light to an arbitrary wavelength light, which arise a problem of increased restriction in wavelength.